This invention relates to either variable-volume or constant volume type central heating and air-conditioning systems that supply conditioned air on a demand basis to a plurality of enclosed spaces to be maintained at desired conditions of temperature, etc. More particularly, the invention relates to an automatic volumetric flow rate control system for regulating a flow restrictor valve that meters the flow of conditioned air through a supply duct in accordance with the temperature condition etc. in the enclosed space serviced by the duct.
In the past, the most common type of air distribution system in central heating and air-conditioning installations has been the constant volume type wherein a uniform volumetric flow of conditioned air is continuously supplied to the various enclosed spaces to be serviced. The air supplied is heated, cooled or otherwise conditioned in accordance with the overall demand imposed on the system. Thus whether or not the temperature condition etc. in an enclosed space requires a certain amount of air circulation to maintain the desired condition, that amount of air circulation is maintained.
This type of system is inefficient from an energy conservation point of view because during many periods of the day and during many seasons of the year the volume of air flow may far exceed that required by the circumstances. Accordingly, in view of the recent increase in emphasis on energy conservation it has become highly advantageous to utilize a variable-volume-type air distribution system so that a minimum load is imposed on the heating and air-conditioning installation.
Variable-volume-type systems utilize flow restrictor valves in the supply ducts to the individual enclosed spaces to be serviced so that when the temperature in an enclosed space is maintained at the desired level by the existing sources of thermal energy, (e.g., radiation from lighting, from energy emitting appliances, from persons in the space etc.), the restrictor valve reduces the flow rate to a minimum level that matches the demand for conditioned air. When the factors effecting the temperature condition in the room are such that heated or cooled air flow must be modulated in order to provide the desired condition, then the flow restrictor valve is adjusted to pass a greater or smaller volume of air through the duct. In the past the regulation of the restrictor valve has been accomplished by manually controlled devices that must be adjusted periodically depending upon the circumstances. Automatic regulation has been impractical because of the many parameters that are involved in the proper adjustment of the volume of air supplied.
The flow rate through a restriction such as a variable orifice may be determined from the following expression: EQU Flow Rate (CFM)= 13.55 d.sup. 2 P
where d is the diameter of the circular orifice and P is the pressure drop across the orifice.
Accordingly, the volumetric flow rate is a function of the square of the orifice diameter and the square root of the pressure differential. The pressure drop across the orifice will vary as the particular condition of the distribution system changes on an hourly and/or seasonal basis due to all the various factors that effect the total demand on the system. Accordingly, at various times of the day and at various seasons of the year the pressure in a particular duct will undergo substantial changes and thus the pressure differential across the orifice cannot be accurately predicted based on the orifice diameter.
The control system of the present invention, however, resolves the difficulties described above and affords other features and advantages heretofore not obtainable.